1. Field of the Invention
The present invention relates to a fluid bearing device and also to a disk drive apparatus having the fluid bearing device.
2. Description of the Related Art
A laser printer has a motor for rotating a polygon mirror. A video cassette recorder (VCR) has a motor for rotating a head drum. A disk drive (a magnetic disk drive, an optical disk drive, or an optomagnetic disk drive) has a motor for rotating a disk (a magnetic disk, an optical disk, or an optomagnetic disk). The motor is one of the most important components of any one of these information apparatuses. It is desired that a rotary machine (e.g., an electric motor) for use in information apparatuses be machined with high precision and have high impact resistance.
Ball bearings have hitherto been used in the rotary machines incorporated in information apparatuses. The balls and/or race of a ball bearing used in a rotary machine will be damaged when it receives an impact. In this case, the shaft supported by the bearing will incline while being driven.
The recent notable trend is the miniaturization of information apparatuses. It is demanded that disk drives, among other things, be made thinner to be more portable. Very thin electric motors for use in disk drives should therefore be developed. An electric motor, the shaft of which is supported by a ball, cannot be made as thin as is desired, because of the structural configuration of the ball bearing.
A fluid bearing is now replacing a ball bearing in the rotary machine incorporated in an information apparatus. The fluid bearing comprises a fixed section and a rotary section. These sections are spaced part, defining a gap. The gap is filled with working fluid such as lubricating oil or air. The surface of at least one of the sections, which opposes the other section, has a groove for guiding the working fluid in a predetermined direction. As the fluid flows in that direction, guided by the groove, dynamic pressure is built up in the gap between the fixed section and the rotary section. The dynamic pressure thus generated holds the rotary section in a prescribed position.
A fluid bearing is regarded as advantageous over a ball bearing in three respects. First, it is less liable to undergo rotational vibration. Second, it is more resistant to impact. Third, it can be designed to be thinner.
FIG. 1 schematically shows a disk drive having an electric motor which incorporates a conventional fluid bearing. As can be seen from FIG. 1, the motor is a so-called "axial-gap type" brushless motor. It comprises a multi-phase armature coil 1 and a permanent magnet 2 which oppose each other. The coil 1 and the magnet 2 are the main components of a brushless motor. The fluid bearing comprises a fixed section 3 (or a shaft 9) and a rotary section 4 (or a cup-shaped member 12). The sections 3 and 4 are spaced apart, defining a thrust bearing gap 5 and a radial bearing gap 6 between them. Both gaps 5 and 6 are filled with working fluid 7 such as lubricating oil or air. A disk 8 is mounted on the rotary section 4 and can be rotated as the section 4 is driven by the electric motor.
FIGS. 2A and 2B show a conventional fluid bearing. As is seen from FIG. 2A, the bearing body includes a shaft 9 (or a fixed section 3) and a cup-shaped member 12 (or a rotary section 4). The cup-shaped member 12 accommodates the shaft 9 such that the thrust bearing surface 10 and axial bearing surface 11 of the shaft 9 are spaced apart from the inner surfaces of the member 2 by predetermined distances, thus forming a thrust bearing gap 5 and a radial bearing gap 6. Both gaps 5 and 6 are filled with working fluid 7. Herringbone grooves 13A and 13B are formed in the radial bearing surface 11 and arranged in two rows, all around the circumference thereof. Each of the herringbone grooves 3A and 13B is generally v-shaped and oriented to the direction in which the shaft 9 rotates. Hence, when the shaft 9 rotates, the working fluid 7 is forced toward the tips of the herringbone grooves 13A and 13B, thus generating a pressure. The pressure imparts radial rigidity to the shaft 9. The pressure also prevents the shaft 9 from inclining, provided that the rows of the herringbone grooves are spaced apart by an appropriate distance.
As shown in FIG. 2B, spiral grooves 14 are formed in the thrust bearing surface 10. When the shaft 9 rotates, the spiral grooves 14 make the working fluid 7 to flow in such a direction that build a thrust load 10 capacitance which causes the shaft 9 to float, overcoming the attraction of the permanent magnet of the axial-gap type motor.
The pressure distributes in the radial and thrust directions of the fluid bearing as indicated by shaded region 15 in FIG. 2A.
The fluid bearing shown in FIGS. 2A and 2B has two problems if it thin enough to be incorporated into an axial-gap type motor. The first problem is that its thrust bearing surface 10, which is proportionally small, may fail to provide a thrust load capacitance great enough to float the rotary section 4 against the attraction of the permanent magnet of the motor. The second problem is that its radial rigidity is insufficient since the grooves made in the radial bearing surface 11 have but a small width L.
FIG. 3 is a diagram illustrating how the radial rigidity of the conventional fluid bearing change with dimensionless quantity L/D, where L is the width of the herringbone grooves 13 and D is the diameter of the shaft 9. As is evident from FIG. 3, when L/D reduces to half the initial value, the radial rigidity decreases a quarter or less the initial value. The radial rigidity is a primary factor of the motor incorporating the fluid bearing, which influences the impact resistance of the disk drive apparatus incorporating the motor. Without decreasing the radial rigidity, it is difficult to make the motor sufficiently thin by the existing technology of manufacturing the axial-gap motors.
As has been indicated, a relatively thin conventional fluid bearing for use in an axial-gap electric motor has two inherent problems. First, since the bearing is made thin, it has no thrust load capacitance large enough to float the rotary section against the attraction of the permanent magnet of the motor. Second, since the herringbone grooves made in the radial bearing surface are narrow, the bearing has but an insufficient radial rigidity.
A fluid bearing of a different type, which is suitable for use in a disk drive apparatus, is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-289577 (hereinafter referred to as "Publication"). In this fluid bearing, a dynamic pressure is generated in the gap between the disk and the enclosure, due to the air flowing through the gap as the disk rotates. The dynamic pressure prevents the disk from inclining to the shaft and provides thrust rigidity. Also, a dynamic pressure is generated also in the gap between the outer circumference of the disk and the enclosure as the disk rotates. This dynamic pressure provides radial rigidity.
Generally, a fluid bearing has but a very low rigidity when made small. To increase the rigidity to a sufficient value, it is necessary to reduce the gap between the rotary and the fixed sections or to increase the viscosity of the working fluid. To reduce the gap it is required that the opposing surfaces of the rotary and the fixed sections be machined with high precision. When the viscosity of the working fluid is increased, the shaft loss will increase, raising power consumption. Hence, in the case where a fluid bearing is incorporated in a disk drive apparatus, it is important that the bearing has as large a bearing area as the space in the apparatus permits. In view of this, the technique of using the disk as bearing surface, as in the bearing disclosed in the Publication, is a good method of proving thrust rigidity and preventing the disk from inclining to the shaft.
The fluid bearing disclosed in the Publication is disadvantageous, however, in the following two respects.
First, the bearing has but small radial rigidity. As indicated, generally a fluid bearing has but a very low rigidity if its bearing surface is narrow (see FIG. 3). In the fluid bearing disclosed in the Publication, the width of the bearing surface is the thickness of the disk. To increase the radial rigidity, the disk must be replaced by a thicker one. The thicker the disk, the longer the time which the disk requires to reach a desired speed and the larger the space which accommodates the disk. Thus, the disk cannot be replaced by a thicker one. Rather, it is desirable that the disk be thinner so that the apparatus may be smaller. In practice it is difficult for the fluid bearing to acquire a sufficient radial rigidity.
Secondly, there is the possibility that the disk contacts the enclosure when the motor is started or stopped. To provide a sufficient thrust rigidity and to reliably prevent the disk from inclining to the shaft, the gap between the disk and the enclosure must be as narrow as possible. If the gap is too narrow, the disk may contact the enclosure as the motor is started or stopped, rotating the disk at a low speed. Rotated at low speeds, the disk vibrates and may contact the enclosure. If this happens, the recording surface of the disk is damaged, inevitably destroying the information recorded on the recording surface. To avoid such destruction of information, that surface of the disk which is used as bearing surface is not used as a recording surface and no recording/reproducing head is located at that surface of the disk. Consequently, even if the disk drive apparatus has two disks, only two disk surfaces contribute to recording, which is disadvantageous from a viewpoint of information-recording density. Furthermore, if the disk contacts the enclosure, its surface will be scratched, unavoidably forming dust. Both the damaged surface and the dust will jeopardize the recording/reproducing operation of the disk drive.